Acids Bases and Salts — Class 10 Chemistry NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 10 Chemistry chapter "Acids Bases and Salts" — 10 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 10 Chemistry chapter "Acids Bases and Salts" — 10 important questions with detailed answers for CBSE board…
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NCERT Solutions for Class 10 Chemistry — Acids Bases and Salts. Step-by-step answers to all 10 textbook questions from this chapter, written for CBSE board preparation and free to use.
Acids Bases and Salts — All 10 Questions Solved
Q1. A solution turns red litmus blue. What is its nature? What will happen if this solution reacts with zinc metal? Write the chemical equation if a reaction occurs.
Step 1: Identify the nature of the solution.
* Acids turn blue litmus red.
* Bases turn red litmus blue.
* Since the solution turns red litmus blue, it is basic in nature.
Step 2: Determine the reaction with zinc metal.
* Bases react with certain active metals (like zinc, aluminium) to produce hydrogen gas and a salt.
* For example, sodium hydroxide (a base) reacts with zinc metal.
Step 3: Write the chemical equation.
* Let's assume the base is Sodium Hydroxide (NaOH).
* Reaction: Sodium hydroxide reacts with zinc metal to produce sodium zincate and hydrogen gas.
* Equation: 2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) + H₂(g)
Q2. When dilute hydrochloric acid is added to: (a) Magnesium ribbon (b) Sodium carbonate Describe the observations in each case and write the balanced chemical equations.
Case (a): Dilute HCl added to Magnesium ribbon
* Observation: Rapid effervescence (formation of bubbles) will be observed. These bubbles are of hydrogen gas, which burns with a 'pop' sound when a burning splinter is brought near it.
* Chemical Equation: Magnesium metal reacts with dilute hydrochloric acid to produce magnesium chloride (a salt) and hydrogen gas.
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Case (b): Dilute HCl added to Sodium carbonate
* Observation: Brisk effervescence (rapid bubbling) will be observed. The gas produced is carbon dioxide, which turns limewater (calcium hydroxide solution) milky.
* Chemical Equation: Sodium carbonate reacts with dilute hydrochloric acid to produce sodium chloride (a salt), water, and carbon dioxide gas.
Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
Q3. A student mixes 10 mL of a strong acid (like HCl) with 10 mL of a strong base (like NaOH) in a test tube. (a) What will be the colour of the resulting solution if a drop of phenolphthalein indicator is added to it? (b) What will be the pH of the resulting solution? (c) Write the chemical equation for the reaction.
Step 1: Understand the reaction.
* When a strong acid (HCl) reacts with a strong base (NaOH), a neutralisation reaction occurs. If equal volumes and concentrations of a strong acid and strong base are mixed, they completely neutralise each other.
Step 2: Determine the colour with phenolphthalein.
* Phenolphthalein indicator is colourless in acidic and neutral solutions but turns pink in basic solutions.
* Since the resulting solution is neutral (due to complete neutralisation), phenolphthalein will remain colourless.
Step 3: Determine the pH of the resulting solution.
* A neutral solution has a pH of 7.
Step 4: Write the chemical equation.
* Hydrochloric acid reacts with sodium hydroxide to form sodium chloride (salt) and water.
* HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Q4. Why is it advised to use an antacid when someone suffers from indigestion? Explain with a suitable chemical reaction.
Step 1: Understand indigestion.
* Our stomach naturally produces hydrochloric acid (HCl) which aids in the digestion of food.
* During indigestion, the stomach produces an excess amount of this acid, leading to discomfort, pain, and irritation (a condition commonly known as acidity).
Step 2: Role of antacids.
* Antacids are mild bases (or alkaline substances). They typically contain compounds like magnesium hydroxide (Mg(OH)₂) or aluminium hydroxide (Al(OH)₃).
Step 3: Chemical reaction and relief.
* When an antacid is consumed, its basic nature allows it to react with and neutralise the excess hydrochloric acid in the stomach.
* This neutralisation reaction reduces the acidity in the stomach, thereby providing relief from the pain and discomfort caused by indigestion.
* Chemical Reaction Example:
Mg(OH)₂(s) (Antacid, mild base) + 2HCl(aq) (Excess stomach acid) → MgCl₂(aq) (Salt) + 2H₂O(l) (Water)
Q5. Why is it recommended to add acid to water, and not water to acid, while diluting a concentrated acid?
Step 1: Understand the nature of dilution.
* The process of diluting a concentrated acid (like concentrated sulphuric acid or nitric acid) is highly exothermic, meaning it releases a large amount of heat.
Step 2: Consequence of adding water to acid.
* If water is added to a concentrated acid, the lighter water will float on the surface of the denser acid. The heat generated by the reaction will be localised and intense. This intense heat can cause the water to rapidly convert into steam. The sudden, violent formation of steam can cause the acid solution to splash out of the container, potentially causing severe burns to the skin, damage to clothing, or injury to the eyes. The glass container might also crack or break due to the excessive local heating.
Step 3: Recommended procedure (acid to water).
* When concentrated acid is added slowly and carefully to a large volume of water while stirring continuously, the heat generated is distributed over the larger volume of water. Water has a high specific heat capacity, so it can absorb a significant amount of heat without a drastic increase in temperature. This controlled heat dissipation prevents dangerous splashing and ensures safety.
Conclusion:
* Therefore, adding acid to water is the safer procedure because it allows the heat produced to be absorbed gradually by the larger volume of water, preventing uncontrolled boiling and splashing.
Q6. What is meant by 'water of crystallisation'? Give two examples of salts that contain water of crystallisation and write their chemical formulae.
Step 1: Define water of crystallisation.
* Water of crystallisation refers to the fixed number of water molecules that are chemically associated with one formula unit of a salt in its crystalline state. These water molecules are an integral part of the crystal structure and are responsible for the specific shape and often the colour of the crystals.
Step 2: Provide examples and chemical formulae.
* Example 1: Copper Sulphate (Blue Vitriol)
* When copper sulphate crystallises from an aqueous solution, it incorporates five water molecules per formula unit.
* Chemical Formula: CuSO₄·5H₂O
* Example 2: Gypsum
* Gypsum is calcium sulphate with two water molecules of crystallisation.
* Chemical Formula: CaSO₄·2H₂O
* Other common examples include:
* Washing Soda (Sodium Carbonate Decahydrate): Na₂CO₃·10H₂O
* Iron Sulphate (Green Vitriol): FeSO₄·7H₂O
Q7. Explain the Chlor-alkali process. Name the main products obtained and write one important use for each.
Step 1: Introduction to the process.
* The Chlor-alkali process is an important industrial method for manufacturing sodium hydroxide (an alkali). It gets its name from 'chlor' for chlorine and 'alkali' for sodium hydroxide, which are the primary products.
* The process involves the electrolysis of an aqueous solution of sodium chloride (commonly known as brine).
Step 2: Describe the process and reactions.
* When electricity is passed through a concentrated aqueous solution of sodium chloride, it decomposes into sodium hydroxide, chlorine gas, and hydrogen gas.
* This reaction takes place in an electrolytic cell.
* At the anode (positive electrode): Chlorine gas is produced.
2Cl⁻(aq) → Cl₂(g) + 2e⁻
* At the cathode (negative electrode): Hydrogen gas is produced.
2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq)
* Near the cathode: Sodium hydroxide solution is formed.
* Overall Reaction:
2NaCl(aq) + 2H₂O(l) $\xrightarrow{\text{Electrical energy}}$ 2NaOH(aq) + Cl₂(g) + H₂(g)
Step 3: List main products and their uses.
* The main products obtained are:
1. Chlorine gas (Cl₂):
* Use: Used in water treatment (as a disinfectant), manufacturing of Polyvinyl Chloride (PVC), and as a bleaching agent in industries.
2. Hydrogen gas (H₂):
* Use: Used as a fuel, in the hydrogenation of oils to make margarine, and in the production of ammonia for fertilisers.
3. Sodium Hydroxide (NaOH):
* Use: Used in the manufacturing of soaps and detergents, paper making, artificial fibres (like rayon), and for degreasing metals.
Q8. Differentiate between baking soda and washing soda on the basis of their chemical composition and one major use. Explain how baking soda is prepared.
Step 1: Differentiate based on chemical composition.
* Baking Soda:
* Chemical Name: Sodium hydrogen carbonate (or Sodium bicarbonate)
* Chemical Formula: NaHCO₃
* Washing Soda:
* Chemical Name: Sodium carbonate decahydrate
* Chemical Formula: Na₂CO₃·10H₂O
Step 2: Differentiate based on one major use.
* Baking Soda:
* Major Use: It is used as an antacid to neutralise excess acid in the stomach during indigestion. It is also used as an ingredient in baking powders for cooking (as a leavening agent) and in soda-acid fire extinguishers.
* Washing Soda:
* Major Use: It is primarily used as a cleaning agent for domestic purposes (like washing clothes). It is also used for removing the permanent hardness of water and in the glass, soap, and paper industries.
Step 3: Explain the preparation of baking soda.
* Baking soda (Sodium hydrogen carbonate) is prepared using the Solvay process (which is also a part of the process for making washing soda).
* It is produced by reacting a cold and concentrated solution of sodium chloride (brine) with ammonia and carbon dioxide.
* Raw materials required: Sodium chloride (NaCl), ammonia (NH₃), carbon dioxide (CO₂), and water (H₂O).
* Chemical Reaction:
NaCl(aq) + NH₃(aq) + H₂O(l) + CO₂(g) → NH₄Cl(aq) + NaHCO₃(s)
(Sodium hydrogen carbonate, being sparingly soluble, precipitates out as a solid.)
Q9. How is Plaster of Paris prepared? Write its chemical formula and one important use. What happens when Plaster of Paris is mixed with water?
Step 1: Preparation of Plaster of Paris.
* Plaster of Paris (POP) is chemically known as calcium sulphate hemihydrate. It is prepared by heating gypsum (calcium sulphate dihydrate, CaSO₄·2H₂O) at a carefully controlled temperature of 373 K (100°C).
* During this heating, gypsum loses three-quarters (1½ molecules) of its water of crystallisation to form Plaster of Paris.
* Chemical Equation:
CaSO₄·2H₂O(s) $\xrightarrow{\text{373 K}}$ CaSO₄·½H₂O(s) + 1½H₂O(g)
(Gypsum) (Plaster of Paris)
* Important Note: The temperature control is crucial. If gypsum is heated above 373 K, it loses all its water of crystallisation and forms anhydrous calcium sulphate (CaSO₄), which is called 'dead burnt plaster'. Dead burnt plaster loses its ability to set with water.
Step 2: Chemical Formula and one important use.
* Chemical Formula: CaSO₄·½H₂O
* One Important Use: It is widely used by doctors for setting fractured bones in the correct position by making plaster casts. It is also used for making toys, decorative materials, and for providing smooth surfaces for walls and ceilings.
Step 3: What happens when Plaster of Paris is mixed with water.
* When Plaster of Paris is mixed with water, it quickly re-hydrates (absorbs water) to form gypsum again. This reaction results in the formation of a hard solid mass within 5 to 15 minutes, which is its characteristic setting property.
* Chemical Equation:
CaSO₄·½H₂O(s) + 1½H₂O(l) → CaSO₄·2H₂O(s)
(Plaster of Paris) (Gypsum)
Q10. A student tests the pH of four salt solutions P, Q, R, and S with a universal indicator. The pH values obtained are 7, 2, 9, and 5 respectively. (a) Which solution is acidic? (b) Which solution is basic? (c) Which solution is neutral? (d) Arrange these solutions in increasing order of their hydrogen ion concentration.
Step 1: Recall the pH scale.
* The pH scale ranges from 0 to 14.
* pH < 7 indicates an acidic solution.
* pH = 7 indicates a neutral solution.
* pH > 7 indicates a basic (or alkaline) solution.
* A lower pH value means higher acidity and, consequently, a higher concentration of hydrogen ions ([H⁺]).
Step 2: Identify acidic, basic, and neutral solutions from the given pH values.
* Given pH values: P=7, Q=2, R=9, S=5
* (a) Acidic solution:
* Solutions with a pH less than 7 are acidic.
* Therefore, Q (pH=2) and S (pH=5) are acidic solutions.
* (b) Basic solution:
* Solutions with a pH greater than 7 are basic.
* Therefore, R (pH=9) is a basic solution.
* (c) Neutral solution:
* Solutions with a pH equal to 7 are neutral.
* Therefore, P (pH=7) is a neutral solution.
Step 3: Arrange these solutions in increasing order of their hydrogen ion concentration.
* Increasing order of hydrogen ion concentration means decreasing order of pH value (i.e., from most basic/least acidic to most acidic/least basic).
* The pH values in increasing order are: 2 (Q), 5 (S), 7 (P), 9 (R).
* To arrange in *increasing order of hydrogen ion concentration*, we need to go from the lowest [H⁺] (highest pH) to the highest [H⁺] (lowest pH).
* So, the increasing order of hydrogen ion concentration is:
R (pH=9) < P (pH=7) < S (pH=5) < Q (pH=2)
Acids Bases and Salts — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Which of the following statements is true about litmus paper?
- A. Blue litmus turns red in a basic solution.
- B. Red litmus turns blue in an acidic solution.
- C. Blue litmus turns red in an acidic solution.
- D. Red litmus turns red in a basic solution.
Answer: C. Blue litmus turns red in an acidic solution. — Acids turn blue litmus paper red, while bases turn red litmus paper blue. Litmus is an indicator used to test the acidic or basic nature of a solution.
MCQ 2. A solution turns red litmus blue. Its pH is likely to be:
- A. 4
- B. 6
- C. 7
- D. 10
Answer: D. 10 — Solutions that turn red litmus blue are basic in nature. Basic solutions have a pH greater than 7.
MCQ 3. When an acid reacts with a metal, which gas is typically produced?
- A. Oxygen
- B. Hydrogen
- C. Carbon dioxide
- D. Nitrogen
Answer: B. Hydrogen — Most active metals react with acids to produce a salt and hydrogen gas. For example, Zn + 2HCl → ZnCl2 + H2.
MCQ 4. The reaction between an acid and a base to form salt and water is known as:
- A. Oxidation reaction
- B. Reduction reaction
- C. Neutralization reaction
- D. Displacement reaction
Answer: C. Neutralization reaction — Neutralization is a chemical reaction in which an acid and a base react quantitatively with each other to form a salt and water.
MCQ 5. What is the chemical formula for washing soda?
- A. NaHCO3
- B. Na2CO3
- C. Na2CO3.10H2O
- D. CaCO3
Answer: C. Na2CO3.10H2O — Washing soda is hydrated sodium carbonate, which has the chemical formula Na2CO3.10H2O, indicating ten molecules of water of crystallization.
MCQ 6. Gypsum has two molecules of water of crystallization. Its chemical formula is:
- A. CaSO4.H2O
- B. CaSO4.2H2O
- C. CaSO4.1/2H2O
- D. CaSO4
Answer: B. CaSO4.2H2O — Gypsum is calcium sulfate dihydrate, meaning it contains two molecules of water of crystallization per formula unit, giving it the formula CaSO4.2H2O.
MCQ 7. Which gas is released when dilute hydrochloric acid is added to calcium carbonate?
- A. Hydrogen
- B. Oxygen
- C. Carbon dioxide
- D. Nitrogen dioxide
Answer: C. Carbon dioxide — Acids react with metal carbonates to produce carbon dioxide gas, a salt, and water. The reaction is: CaCO3 + 2HCl → CaCl2 + H2O + CO2.
MCQ 8. Which of the following is a strong acid?
- A. Acetic acid (CH3COOH)
- B. Citric acid
- C. Hydrochloric acid (HCl)
- D. Carbonic acid (H2CO3)
Answer: C. Hydrochloric acid (HCl) — Hydrochloric acid is a strong acid because it completely ionizes in water. Acetic acid, citric acid, and carbonic acid are examples of weak acids.
MCQ 9. Which of these substances can be used as an olfactory indicator?
- A. Litmus paper
- B. Phenolphthalein
- C. Onion
- D. Methyl orange
Answer: C. Onion — Olfactory indicators change their smell in acidic or basic media. Onion, vanilla essence, and clove oil are common examples of olfactory indicators.
MCQ 10. Which of the following is commonly used as an antacid?
- A. Sodium chloride
- B. Magnesium hydroxide
- C. Acetic acid
- D. Sulfuric acid
Answer: B. Magnesium hydroxide — Magnesium hydroxide (milk of magnesia) is a mild base commonly used as an antacid to neutralize excess stomach acid and relieve indigestion.
MCQ 11. Tooth decay starts when the pH of the mouth falls below:
- A. 7.0
- B. 6.5
- C. 5.5
- D. 4.0
Answer: C. 5.5 — Bacteria in the mouth produce acids by degrading sugar and food particles. When the pH of the mouth falls below 5.5, the enamel (calcium phosphate) starts to corrode, leading to tooth decay.
MCQ 12. What is the chemical name of Plaster of Paris?
- A. Calcium sulphate dihydrate
- B. Calcium chloride
- C. Calcium sulphate hemihydrate
- D. Sodium bicarbonate
Answer: C. Calcium sulphate hemihydrate — Plaster of Paris is chemically known as calcium sulphate hemihydrate, with the formula CaSO4.1/2H2O. It is prepared by heating gypsum.
MCQ 13. When zinc reacts with sodium hydroxide solution, it produces hydrogen gas and:
- A. Sodium zincate
- B. Zinc oxide
- C. Sodium chloride
- D. Zinc hydroxide
Answer: A. Sodium zincate — Certain metals like zinc and aluminium react with strong bases like NaOH to produce hydrogen gas and a salt. The salt formed in this reaction is sodium zincate (Na2ZnO2).
MCQ 14. The pH of pure water at 25°C is:
- A. 0
- B. 7
- C. 14
- D. Slightly less than 7
Answer: B. 7 — Pure water is neutral, meaning it is neither acidic nor basic. Therefore, its pH value is 7 at standard temperature (25°C).
MCQ 15. When baking soda is heated, what gas is evolved?
- A. Carbon monoxide
- B. Oxygen
- C. Carbon dioxide
- D. Hydrogen
Answer: C. Carbon dioxide — Baking soda (sodium bicarbonate, NaHCO3) decomposes upon heating to produce sodium carbonate, water, and carbon dioxide gas. This CO2 makes cakes and bread fluffy.
MCQ 16. Which of the following is a strong base?
- A. Magnesium hydroxide (Mg(OH)2)
- B. Ammonium hydroxide (NH4OH)
- C. Calcium hydroxide (Ca(OH)2)
- D. Potassium hydroxide (KOH)
Answer: D. Potassium hydroxide (KOH) — Potassium hydroxide (KOH) is a strong base because it completely dissociates in water. Magnesium hydroxide, ammonium hydroxide, and calcium hydroxide are considered weak bases for Class 10 curriculum.
MCQ 17. Which of the following salts will give an acidic solution when dissolved in water?
- A. Sodium chloride (NaCl)
- B. Potassium nitrate (KNO3)
- C. Ammonium chloride (NH4Cl)
- D. Sodium acetate (CH3COONa)
Answer: C. Ammonium chloride (NH4Cl) — Ammonium chloride is formed from a strong acid (HCl) and a weak base (NH4OH). In solution, the ammonium ion hydrolyzes to produce H+ ions, making the solution acidic.
MCQ 18. Acid rain has a pH value generally:
- A. Above 7.0
- B. Between 5.6 and 7.0
- C. Below 5.6
- D. Exactly 7.0
Answer: C. Below 5.6 — Normal rain has a pH of approximately 5.6 due to dissolved CO2. Rain with a pH below 5.6 is considered acid rain because of additional acidic pollutants.
MCQ 19. Adding water to a concentrated acid will:
- A. Increase its pH
- B. Decrease its pH
- C. Not change its pH
- D. Make it more acidic
Answer: A. Increase its pH — Diluting an acid reduces the concentration of H+ ions, making the solution less acidic and thus increasing its pH towards 7.
MCQ 20. Bleaching powder is produced by the reaction of chlorine gas with:
- A. Slaked lime
- B. Quicklime
- C. Limestone
- D. Calcium carbonate
Answer: A. Slaked lime — Bleaching powder (Calcium oxychloride, CaOCl2) is prepared by passing chlorine gas over dry slaked lime (calcium hydroxide, Ca(OH)2). The reaction is: Ca(OH)2 + Cl2 → CaOCl2 + H2O.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.